How to include the document id in Firestore collection in Angular 5

As of May, 10, 2019 you can now use:

valueChanges({idField?: string})

and pass in an optional field name that will contain the id of the document.

Such as in your original post:

get_the_posts(){
    this.Post_collection = this.afs.collection('posts'); 
    return this.Posts = this.Post_collection.valueChanges({ idField: 'id' }); 
}

This would indeed return your objects in your desired output.

From the Collections documentation:

Streaming collection data

There are multiple ways of streaming collection data from Firestore.

valueChanges({idField?: string})

What is it? - The current state of your collection. Returns an Observable of data as a synchronized array of JSON objects. All Snapshot metadata is stripped and just the document data is included. Optionally, you can pass an options object with an idField key containing a string. If provided, the returned JSON objects will include their document ID mapped to a property with the name provided by idField.

[Emphasis mine]

.valueChanges() returns only data without any meta data. you can use .snapshotChanges() for that

this.Post_collection = this.afs.collection('posts');
return this.Posts = this.Post_collection.snapshotChanges().map(actions => {
  return actions.map(a => {
    const data = a.payload.doc.data();
    const id = a.payload.doc.id;
    return { id, ...data };
  });
});

also import import { Observable } from 'rxjs/Observable';

For your second question how to add id when pushing data to firestore. try

//get id from firestore
    let id = this.afs.createId();

    this.post = {
      id:id,
      name:'name',
      description:'description'
    }

    this.afs.collection('posts').doc(id).set(this.post).then();

Now you can use .valueChanges() to get the data.

I got so annoyed that there isn't a method for this, so I made one to share with everyone. Something this basic should be built into the the library itself.

getCollectionWithIDs(collection,key,comparison,value) {
    return this.afs.collection<any>(collection, ref => 
        ref.where(
            key, comparison, value
        )).snapshotChanges().pipe(
            map(actions => actions.map(a => {
            const data = a.payload.doc.data();
            const id = a.payload.doc.id;
            return { id, ...data };
        })))
}

Note that this.afs is type AngularFirestore that should be in the constructor.